A species changes color over time if its environment changes. Many other types of changes can take place, too. The darker deer mice evolved into the lighter deer mice in some locations. What do you think changed about the environment?

Biological Evolution

Adaptations in a species add up over time. These adaptations will mean that many genes have changed. If the environment is stable, the species won't change. But if the environment is changing, the species will need to adapt. Many adaptations may be necessary. In time, the species may change a lot. The descendants will be very different from their ancestors. They may even become a new species. This process is called biological evolution but is usually referred to just as
evolution
.

Organisms alive today evolved from earlier life forms. We can learn about this from fossils. For example, horse fossils from 60 million years ago are very different from modern horses. Ancient horses were much smaller than they are today (
Figure
below
). The horses’ teeth and hooves have also changed. The horses evolved because of changes in their environment.

Ancient horses were quite different from present-day horses.

Although basic horse evolution appears to be linear, there were many more branches. Horse evolution is very well understood.

Evolution happens because of
natural selection
. Good traits become more common in a population. Bad traits become less common. The deer mouse, species
Peromyscus maniculatus
gives an example of evolution by natural selection. In Nebraska, this mouse is typically brown. But in places where glaciers dropped lighter sand over the darker soil, the mice are light. Why? Because predators could more easily spot the dark mice on light sand. The lighter mice were more likely to survive and have offspring. Natural selection favored the light mice. Over time, the population became light colored. Enough changes may take place over time that the two types of mice become different species.

Vocabulary

evolution
: Change through time. The change in the genetic makeup of a population of organisms over time such that a new species is often the result.

natural selection
: Mechanism for evolution.

Summary

Changes in the genes of a species may result in a new species. This is biological evolution.

Natural selection can change a species or even make a new species. Natural processes favor some traits over others in a population. This causes those traits to be more common in subsequent generations.